1. Technical Field of the Invention
The present invention relates to a steering apparatus for a vehicle that orients road wheels in response to operator input in which the road wheels are not mechanically coupled to the steering wheel.
2. Background and Summary of the Present Invention
A typical automotive vehicle is steered by transmitting the rotation of a steering wheel to a steering mechanism, which directs road wheels in a corresponding fashion. Generally, the steering wheel is located inside the vehicle passenger compartment, and the road wheels are located at the front of the vehicle. Thus, a suitable steering mechanism is necessary to couple the steering wheel and the road wheels.
A representative steering mechanism is a rack-and-pinion type steering mechanism. In a rack-and-pinion steering mechanism, the rotational motion of the steering wheel is communicated through a steering shaft to a pinion gear at its distal end. The pinion gear is engaged with a rack gear disposed laterally between the road wheels, which in turn are coupled to the rack gear by tie rods. In this manner, rotation of the steering wheel is translated into the lateral movement of the rack gear, which causes the road wheels to pivot in the desired direction. In general, mechanical steering mechanisms are power-assisted by hydraulic or electrical assist units.
Mechanical steering mechanisms such as described above have a number of limitations. As the steering wheel and the steering mechanism are mechanically coupled in some fashion, the position of the steering wheel is limited within the vehicle passenger compartment. Moreover, the size and weight of the coupling members limits the layout and performance of the vehicle.
In order to overcome such limitations, it has been proposed to utilize a steering system in which the steering wheel is not mechanically coupled to the road wheels and the road wheels and steering movement is achieved by an electrically controlled motor, for example, a steer-by-wire system. In a steer-by-wire system, a steering actuator operates in response to detected values of various steering parameters, such as steering wheel angle and vehicle speed. The detected values are communicated electronically to the steering actuator from sensors, whereby the steering actuator orients the road wheels in the desired direction.
Steer-by-wire systems solve a number of problems presented above. In addition, there are a number of other advantages innate to steer-by-wire systems that were not apparent in its mechanically coupled counterpart. For example, a steer-by-wire steering system can be integrated into other electronically controlled systems to increase the efficiency and performance of the vehicle.
Although a steer-by-wire system presents distinct advantages, it also presents a number of problems. Since there is no direct mechanical coupling between the operator and the road wheels, it is not necessary for the orientation of the steering wheel to correspond to the orientation of the road wheels. For example, it is possible that the steering wheel could be directed in a left-turn orientation while the road wheels are directed in a right-turn orientation. Such a discrepancy may arise when the steer-by-wire system is powered-down, as is the case when the vehicle is turned off, and the steering wheel is turned without a corresponding pivoting of the road wheels.
Consequently, there is a need in the art for an improved steer-by-wire system that is adapted to correct for misalignment between the steering wheel and the road wheel, and which corrects the misalignment automatically. Furthermore, the steer-by-wire system should be able to correct for misalignment in an efficient and timely manner, such that the steering wheel travels a minimum angular distance from an initial position to the corrected position.
Accordingly, the present invention provides an improved steer-by-wire system comprising a road wheel angle sensor, a steering wheel angle sensor, and a torque feedback actuator. The aforementioned components are coupled to a controller that is adapted to calculate a corrected steering wheel angle based upon the relative angular positions of the road wheels and the steering wheel as measured by the respective sensors. The controller then controls the torque feedback generator to rotate the steering wheel into a corrected position such that the torque feedback generator rotates the steering wheel a minimum angle. During the initialization, the rotational motion of the steering wheel is also controlled so that the rotation is smooth and in an acceptable low speed to ensure the safety of the driver. By checking the smoothness and the progression of the rotational motion, the controller will detect any resistance to the correction that is greater than the anticipated rotational friction. Such resistance includes interference from the vehicle driver. In response to any resistance, the controller is further adapted to issue a warning signal.